The present invention relates to a planar lighting device used for indoor and outdoor illumination or used as a backlight for liquid crystal display panels, advertising panels, advertising towers, and other advertising signs, and comprising a light guide plate that diffuses light emitted from light sources and emits the diffused illuminating light from a light exit surface.
Generally, a liquid crystal display device has a backlight unit to illuminate its liquid crystal display (LCD) panel from the rear side thereof. Currently, large-sized liquid crystal display televisions predominantly use a backlight unit of a type that comprises cold cathode tubes provided behind the rear side of a liquid crystal display panel as light sources to illuminate the liquid crystal display panel (the type will be referred to below as “direct illumination type”; see JP 05-4133 U, for example). The cold cathode tubes are arranged in a housing with white inner walls serving as reflecting surfaces. To achieve a uniform light amount distribution with this type of backlight unit, however, the liquid crystal display panel needs to have a thickness of about 30 mm in a direction perpendicular to the panel as dictated by the principle.
Recent years have been seeing demands for thinner and larger liquid crystal displays and/or liquid crystal displays consuming less power. However, there was a limit to how thin a liquid crystal display could be made because of unevenness in light amount distribution that may occur when the thickness of a backlight unit of the direct illumination type mentioned above used therein had a thickness reduced to 10 mm or less.
As a backlight unit that is better suited to achieve reduction of the thickness, there is a backlight unit of a so-called tandem type that has units provided beside illuminating light sources, respectively (see JP 11-288611 A, for example). A conventional backlight unit of this type could be made somewhat thin to a certain extent by allowing light to be incident from lateral sides of the light guide plate. However, this conventional technology had a shortcoming of a lower light use efficiency than the backlight unit of the direct illumination type and therefore entails more power consumption in order to emit sufficient light for high brightness display.
Thus, light guide plates of various configurations have been proposed to achieve thinner and larger liquid crystal displays or liquid crystal displays consuming less power (see JP 09-304623 A, JP 08-62426 A, JP 10-133027 A, JP 05-249320 A, JP 2001-42327 A, and JP 2005-234397 A).
The area light source device (backlight unit) described in JP 09-304623 A is so formed that after fluorescent lamps are accommodated in a light guide plate, a reflective sheet is disposed on the rear surface of the light guide plate, and the light guide plate is provided on its light exit surface with layers consisting of a transmitted light amount correction sheet, a light diffusing plate, and a prism sheet.
The light guide plate has a substantially rectangular shape and is made of a resin into which fine particles functioning to diffuse illuminating light are dispersed. Also, the light guide plate has a flat upper surface that serves as the light exit surface. Further, grooves each having a U shape in cross-section are formed in the rear surface (surface opposite from the light exit surface) of the light guide plate to accommodate the respective fluorescent lamps. A light amount correction surface to facilitate the emission of illuminating light is formed on the light exit surface of the light guide plate, except in the areas where the fluorescent lamps are located just below.
Thus, JP 09-304623 A describes that the fine particles are mixed into the resin to form the light guide plate, and the emission of the illuminating light is facilitated by the light amount correction surface formed on a part or all of the light exit surface except in the areas where the fluorescent lamps are located just below, making it possible to reduce the total thickness and unnatural unevenness in brightness of the light emitted from the light guide plate.
JP 08-62426 A describes a light guide plate comprising a rectangular illumination surface, a groove with a rectangular cross-section that is formed at the center of the shorter sides of the rectangular illumination surface and in parallel with the longer sides thereof to accommodate a light source, and rear surfaces formed such that a plate thickness gradually decreases from the groove toward both lateral end surfaces on the longer sides to provide a backlight unit for a liquid crystal display device which enables reduction in size, weight, and thickness of a liquid crystal display device as well as reduction in manufacturing costs and power consumption without reducing the amount of illumination provided by the backlight unit (backlight amount).
JP 10-133027 A describes a light guide member (light guide plate) having a groove of a parabolic shape in cross-section taken along a width direction of the light guide member where a light source is arranged, the major axis of the parabolic shape lying in the direction of depth of the groove, in order to obtain a backlight unit that, with a high light use efficiency and, hence, high brightness, makes possible a liquid crystal display device with a thin frame and a reduced overall thickness.
JP 05-249320 A describes a light guide plate in which plate-shape light wave guide layers are laminated on symmetrically inclined high-reflectance layers so as to achieve successively increasing refractive indices and light emitted from light exit end surfaces of the reflective layers illuminates light diffusion layers brightly in order to keep brightness in a display surface of a display panel uniform and illuminate with high brightness. The recess for accommodating a light source is triangular in cross-section.
JP 2001-42327 A describes a large-area, high brightness rear illumination capable of highly uniform illumination by using light guide plates arranged side by side and a given number of linear light sources provided between the light guide plates to achieve an improved liquid crystal backlight unit employed in large-sized liquid crystal displays for wall-hung televisions.
The light guide plates described in the above references aim to achieve a thinner design, a larger design, and/or less power consumption for liquid crystal display devices. However, each of the light guide plates has at least one groove formed at the center thereof to accommodate a linear light source and the thickness of the plate preferably decreases from the groove toward or the end surfaces or surfaces to achieve a thin design.
The light guide plate described in JP 09-304623 A has a light amount correction surface, such as a rough surface or a microprism surface, formed on the light exit surface except in the areas where light sources (fluorescent lamps) are provided just below, to facilitate the emission of the illuminating light incident on the light exit surface at an angle not less than a critical angle. However, this configuration is provided only a limited measure of increase in brightness of illuminating light emitted from the light guide plate provided with the light amount correction surface as compared with illuminating light emitted from a light guide plate without a light amount correction surface. Thus, the light guide plate described in JP 09-304623 A has a shortcoming that the improvement in brightness of illuminating light achieved by providing the light amount correction surface is not significant, the use efficiency of the light emitted from the light sources is low, and the diffusion of the light from the light sources is inadequate, failing to emit uniform and bright enough light from the light exit surface.
The light guide plate described in JP 09-304623 A has the light sources, i.e., fluorescent lamps, fitted in the grooves each with a circular cross-section, whereby the brightness still peaked at the locations above the light sources. Thus, for this light guide plate to be used as an area light source device, unnatural unevenness in brightness observed at the light exit surface must be eliminated using, for example, a transmitted light amount correction sheet, a light diffusing plate, and a prism sheet provided on the side of the light guide plate closer to the light exit surface. This, however, results in increased costs for the manufacture of the area light source device.
With the backlight unit described in JP 08-62426 A, space beneath the inclined rear surfaces of the light guide plate is used to house parts mounted on electronic circuit boards, thereby to provide a low-cost, low power-consumption backlight unit and achieve a smaller, thinner, and lighter design of a liquid crystal display device. No consideration, however, is given to unevenness in brightness of the illuminating light emitted from the light exit surface of the light guide plate.
The backlight unit for a liquid crystal display device described in JP 10-133027 A has the groove with the parabolic cross-section formed in the light guide member (light guide plate) to admit light to the light guide member such that the diffusion in the light guide plate is substantially uniform in order to enhance the light use efficiency. No consideration, however, is given to unevenness of the light emitted from the light exit surface of the light guide member.
The light guide plate described in JP 05-249320 A has a complicated structure with the laminated light wave guide plates to reduce attenuation of brightness as compared with the prior art and achieve uniform brightness, thereby offering enhanced illumination effects. However, the proposed light guide plate has a drawback of increased manufacturing costs.
With the light guide plate described in JP 2001-042327 A, the brightness rises in areas located just above the linear light sources. Therefore, light transmittance inhibiting patterns must be formed to restrict the transmittance of the light from the linear light sources. Further, since the light emitted from the linear light sources travels inside the light guide plate from one end thereof to the other in a plane direction parallel to a light exit surface of the light guide plate, the amount of light attenuates gradually. Thus, the light guide plate described in JP 2001-042327 A can not adequately attain an improved level of brightness.
To solve the above problems, the inventors of the present invention have proposed a planar lighting device using a light guide plate as described in JP 2005-234397 A. The light guide plate is transparent and comprises a rectangular light exit surface, a thick portion positioned at substantially a center of the rectangular light exit surface in parallel with a side of the rectangular light exit surface, thin edge portions formed parallel to the thick portion, and inclined rear portions having a parallel groove formed substantially at a center of the thick portion parallel to the side to accommodate a rod-shaped light source. The inclined rear portions are symmetrical with respect to a plane including the central axis of the rod-shaped light source and perpendicular to the rectangular light exit surface. The inclined rear portions have a thickness that decreases from the thick portion toward the thin edge portions in a direction perpendicular to the side of the rectangular light exit surface to form inclined surfaces on both sides of the parallel groove. The parallel groove narrows toward the rectangular light exit surface until the parallel groove forms an end point symmetrically with respect to a center line of the parallel groove perpendicular to the rectangular light exit surface in a cross-sectional shape of the parallel groove in the direction perpendicular to the rectangular light exit surface in accordance with a ratio of a peak value of illuminance or brightness of light emitted from the rod-shaped light source accommodated in the parallel groove measured at a first portion of the rectangular light exit surface corresponding to the parallel groove to an average value of the illuminance or brightness of the emitted light measured at second portions corresponding to the inclined rear portions.